The present invention relates to a control of downshift to an engine brake running range.
A known automatic transmission comprises a main gearing drivingly connected to an engine and an auxiliary gearing drivingly connected to the main gearing. The main gearing includes a first frictional device, namely an overrunning clutch, and a one-way clutch arranged in parallel to this clutch, while the auxiliary gearing includes a second frictional device, namely a direct clutch, a third frictional device, namely a reduction brake, and a one-way clutch arranged in parallel to the reduction brake.
An automatic transaxle of the A140L type is known and described on pages 4-12 to 4-31 of a service manual entitled "TOYOTA CARINA" issued by Toyota Motor Company in August 1985. It comprises a main gearing including a front planetary gear set and a rear planetary gear set, and an auxiliary gearing including an overdrive planetary gear set. The front planetary carrier, the rear planetary ring gear, and the overdrive planetary carrier are all splined to an intermediate shaft for unitary rotation therewith. The front planetary sun gear and the rear planetary sun gear are common and thus rotatable as a unit. The overdrive planetary ring gear is splined to a counter drive gear that serves as an output shaft. The main gearing includes a first frictional device, namely a second coasting brake, and a one-way clutch arranged in parallel to the second coasting brake. The auxiliary gearing includes a second frictional device, namely an overdrive brake, a third frictional device, namely an overdrive clutch, and a one-way clutch arranged in parallel to the overdrive clutch. The auxiliary gearing is shiftable from a high gear position thereof to a low gear position thereof owing to disengagement of the second frictional device and engagement of the third frictional device. A 2-3 shift valve has a fluid line communicating with the second coasting brake for initiating engagement thereof, while a 3-4 shift valve has a fluid line communicating with the overdrive direct clutch for initiating engagement thereof. The fluid line between the 1-2 shift valve and the second coasting brake is not provided with any orifice, while the fluid line between the 3-4 shift valve, and the overdrive direct clutch is provided with a one-way orifice and an accumulator. A II range hydraulic pressure acts on the 1-2 and 3-4 shift valves when a manual valve is set to an engine brake running range, namely a II range, causing the 1-2 shift valve to allow a supply of hydraulic fluid to the second coasting brake and 2-3 shift valve to allow supply of hydraulic fluid via the one-way orifice and the accumulator to the overdrive direct clutch. Thus, during this transient period, owing to the provision of the one-way orifice and the accumulator, the second coasting brake is engaged prior to engagement of the overdrive direct clutch. Engagement of the second coasting brake causes a drop in revolution speed of the intermediate shaft, increasing a relative rotation between the overdrive planetary carrier splined to the intermediate shaft and the overdrive planetary sun gear that has been released by disengagement of the overdrive brake, since the rotational speed of the overdrive planetary ring gear is unchanged. Since the overdrive direct clutch has to connect the overdrive planetary carrier with the overdrive planetary sun gear, it is very difficult to alleviate shock taking place during engagement of the overdrive direct clutch and cope with endurance problem of the overdrive direct clutch.
An object of the present invention is to provide a control of downshift to an engine brake running range which does not cause shock an endurance problem like that encountered in the prior art.